Piezoelectric crystal filter circuit



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PIEZOELECTRIC CRYSTAL FILTER CIRCUIT Filed Jan. 7, 1939 ZEAMPL. 7 (9CONVERTER 4 r g :1

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m TER "VAN B. ROBERTS ATTORNEY.

Patented Apr. 21, 1942 2,2so,sos V PIEZOELECTRIC cars'rar. rwrnn cmcurrWalter van B. Roberts, Princeton, N. J., aaalgnor to Radio Corporationof America, a corporation of Delaware Application January 7, 1939,Serial No. 249,151

2 Claims.

My present invention relates to signal translating systems, and moreparticularly to systems including, in combination, an amplifyingcircuit, a sharply resonant system connected to cause degenerativefeedback in said circuit, and means for controlling the degeneration atfrequencies other than the resonant frequency of said resonant system.

The main object of the invention is to provide an amplifying systemhaving normal gain at a single frequency and a lesser gain at otherfrequencies, said other frequencies including frequencies closelyadjacent to said single frequency.

Another important object of my invention is to provide an amplifierwhose gain is substantially uniform over a range of frequencies exceptfor an extremely narrow range of frequencies located within or adjacentsaid first range, the

gain in said narrow range rising to a value having a predetermined ratioto said uniform value.

Still another object of the invention is to provide a signal translatingsystem having a peak value of gain at one frequency and a minimum valueof gain at a closely adjacent frequency, the

gain at other frequencies being of a substantially uniform value ofpredetermined adjustable relation to said peak value over a range offrequencies that is wide relative to the range of frequencies at whichthe gain is substantially r greater than said uniform value.

One important use for such an amplifying system, as described in thisapplication, is in connection with the intermediate frequency (1. F.)

amplifier of a superheterodyne receiver for the purpose of exaggeratingthe amplification of the carrier frequency while maintaining uniform theamplification of the side band frequencies. Such exaggeration is usefulfor reducing the distortion that often accompanies fading of the signalsas a result of the carrier fading down more than the side frequencies, aphenomenon termed "selective fading" and that results in the same typeof distortion as produced by over-modulation. Other uses for such anexaggerated carrier will be apparent in connection with manual sharptuning by reference to indicator devices known under the trade-mark"Magic Eye"; with automatic sharp tuning devices operating on carrierresponse; and, also, in connection with muting devices and so on. Thenovel features which I believe to be characteristic of my invention areset forth in par ticularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

In the drawing:

Fig. 1 shows an amplifier circuit arrangement according to theinvention,

Fig. 2 shows the equivalent circuit of the frequency-selecting elementsof Fig. 1,

Fig. 3 is a diagram showing the variation with frequency of theadmittance of portions of the network shown in Fig. 2.

Fig. 4 shows the variation of the admittance of the circuit of Fig. 2with normal adjustment, and

Fig. 5 shows the variation of the admittance of the circuit of Fig. 2with another adjustment.

Before describing the behaviour of the circuit of Fig. 1 it should benoted that the network located between ground and the cathode of tube Vproduces a degenerative feedback of a magnitude determined by theimpedance of the network. It is well known that the gain of the stage isreduced by this feedback so that the variation of gain of the stagedepends on the way this impedance varies. For this reason it ispreferable to study the behaviour of the degenerative network beforeattempting to describe the complete operation of the invention. In thecathode circuit of tube V in Fig. 1 there is shown a condenser C andcoil L connected in parallel. While not customarily indicated on thedrawing, it will be understood that these elements are not in practiceentirely devoid of resistance. However, it can be shown that theirparallel impedance is the same as that of a resistance-less condenser inparallel with a resistance-less coil,

and, also, in parallel with a pure resistance. Furthermore, if theactual coil and condenser are reasonably low-loss" the equivalentloss-free elements will be of nearly the same values, while theequivalent shunt resistance will be of very high value and all of theequivalent elements will be substantially constant over a considerablerange of frequencies. The equivalent shunt resistance may thus beconsidered as part of the adjustable resistance element R in the cathodecircuit of tube V of Fig. 1. As to thecrystal X itself, its impedance iswell known to be exactly the same as that of a series circuit includingresistance, capacity and inductance, said series circuit being shuntedby a small capacity. In Fig. 2 the series circuit of the crystal is r,Lx, Cx

while the shunt capacity is considered as a part of the "adjustablecondenser C. Thus, finally, the I. F. amplifier network of Fig. 1 isreplaced by the equivalent network of Fig. 2 which is of a form morereadily adapted to analysis.

Refer, now. to Fig. 3 which is a complex plane representation of thevariation with frequency of the admittance of portions of the network ofFig. 2. In Fig. 3 the origin is at point and Conductance values areplotted along the real axis while Susceptance is plotted upwards. It maybe readily shown that the circle represents the admittance of theelements R, r, Lx and C; as

the frequency varies from zero to infinity. At

zero frequency the admittance is a pure conductance HR, and isrepresented by the point a on the circle. As the frequency is increasedthe admittance value moves clockwise around the circle until at thefrequency at which L: and C): resonate the admittance is again a pureconductance l/r plus HR. and is represented by point b. It will be notedthat the diameter of the circle is l/r regardless of the value chosenfor R. As' the frequency is further increased to infinity the admittancevalue continues clockwise around the circle back toward the point a. Sofar there has been considered only the admittance of the resistance andseries resonant portions of the network. As to elements C and L sincethese are loss-free, their admittances are at all frequencies puresusceptances and hence are always located on the vertical axis. Asfrequency increases from zero to infinity the sum of these twosusceptances travels up the verticalaxis from minus infinity to plusinfinity. The total admittance of the network of Fig. 2 is obtained atany frequency by adding the admittance represented by the appropriatepoint on the circle to the susceptance represented by the correspondingpoint on the vertical axis. The total impedance of the network is thenthe reciprocal of the result.

Let us suppose that C is so adjusted that the anti-resonant frequency ofL and C is the same as the series-resonant frequency of and Cx. In thiscase the susceptance of the LC combination vanishes at the samefrequency at which the admittance of the rest of the network becomes apure conductance represented by point b. Furthermore, if any ordinarysized coil and condenser are used at L and C the admittance of these twoelements will be found to remain relatively very small throughout thenarrow range of frequencies in which the admittance of the otherelements traverses nearly the whole circumference of the circle. Hence,the admittance of the entire network has an absolute value that varieswith frequency somewhat as shown in Fig. 4, in which the solid curvecorresponds to a large value of R while the dotted curve is obtainedfrom a smaller value of- R. By making the value of R small enough therelative exaggeration of a peak substantially as before but at otherfre- I at one' frequency where the susceptance of the crystal branch isequal and opposite to that of the LC combination. At this latterfrequency the total admittance drops to substantially I/R. Fig. 5 showsqualitatively the effect of making C a" little larger than the resonantvalue. A similar curve but with the dip on the other side of the crystalfrequency is obtained bymaking C a little too small. Again the relativemagnitudes of the departures from the mean value can be reduced byreducing R. The type of response curve shown in Fig. 5 is sometimesuseful when a single frequency closely adjacent a desired carrier is tobe rejected. The frequency at which this rejection occurs is controlledby the setting of C.

Returning, now, to Fig. 1 there is shown in schematic manner asuperheterodyne receiver system comprising a converter I whose variablytuned signal input circuit 2 may be coupled to any desired signalsource. For example, the usual signal" collector may be employed. andthe signals can be amplified in one or more amplifiers prior toimpression of the signal energy on circuit I. The numeral 3 denotes theusual adjustably tuned local oscillator tank circuit which is tuned overa frequency range differing from the signal range by the I. F. value. Acommon tuner 4, well known to those skilled in the art, is employed toadjust the tuning reactances of circuits 2 and 3. The output circuit 4is fixedly resonated to the desired I. F., while the input circuit I ofI. F. amplifier V is tuned to the same I. F. value.

Amplifier V may be of any desired type; its cathode is grounded throughthe piezo-electric crystal element X. The latter is shunted by theadjustable resistor R; the anti-resonant combination LC shunts thecrystal X as well. Normal negative bias is applied to signal grid 6 fromany desired negative bias source through the resistor I. If AVC biasisused at this point, the phenomena heretofore described becomecomplicated by the fact that in the case of strong signals, which resultin a large AVC voltage, the mutual conductance of tube V is greatlyreduced. This in turn greatly reduces the relative magnitude of theefiects of the degenerative feedback.

'This fact may be usefully employed to provide a carrier-emphasizingaction only on weak signals such as are particularly benefltted by suchaction. The AVC bias may be dreived from the I. F. signals in anydesired and well known manner; for example, a rectifier can be coupledto the output of circuit 8 or 9, and the direct current voltage outputof the rectifier can be applied as the bias for grid 6. The amplifierplate circuit includes the resonant network 8 tuned-to the I. F., andthe circuit 9 may be coupled thereto to transfer the I. F. energy to asecond detector. Of course, if

desired, the I. F. circuit 9 cache coupled to additional I. F.amplifiers prior to demodulation of the I. F. energy. Any type of audioamplifier and reproducer can be used subsequent to the demodulator.

The amplifier V is a typical amplifier of intermediate frequency energyexcept for the inclusion of the degenerative network in the cathodecircuit. Transformers T and T1 are bandpass transformers. The gain-ofthe stage is the more reduced from normal the higher the impedance ofthe cathode network.- Hence, the gain is greater the larger theadmittance of this network. Therefore, the curves of Figs. 4 and 5 alsorepresent the stage gain in the-vicinity of crystal resonance.Frequencies outside of the band passed by the transformers T, however,are attenuated by these transformers regardless of the state ofdegeneration so that the slight rise'of the curves of Figs. 4 and 5 asthe frequency departs widely from the carrier frequency has very littleeffect on the over-all selectivity characteristic of the amplifierstage.

To recapitulate the performance of the system:

(1) With C tuned to I. F. resonance, and R very small, the gain issubstantially normal; as R is increased the side frequency gain isdecreased while the carrier frequency gain is reduced only to a slightextent depending on the equivalent series resistance of the crystal.Thus the carrier may be exaggerated to any desired extent while the gainthroughout the side band frequencies is substantially uniform.

(2) With C suitably detuned not only is the carrier emphasized, but afrequency determined by the amount of'detuning is suppressed. In thiscase. also, the amount of emphasis and suppression is determined by theadjustment of R.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In an amplifier of modulated carrier waves,

a tube having at least a cathode, control electrode and outputelectrode, a modulated carrier output circuit coupled to the outputelectrode of said tube, a network for producing exaggeration of thecarrier with respect to the modulation side band frequency components,said network comprising a parallel resonant cicuit, tuned to the carrierfrequency of applied carrier waves, arranged inthe space current path ofsaidtube between said cathode and a point of invariable potential, saidresonant circuit being of substantially low resistance, a piezo-electriccrystal connected in shunt with said resonant circuit and tuned to saidcarrier frequency, a purely resistive element, connected between saidcathode and said point, arranged in shunt with each of said crystal andresonant circuit, a resonant modulated carrier input circuit connectedbetween said control electrode and said point whereby voltages of saidmodulation side band frequency components developed across saidexaggeration network are applied in substantially uniform manner uponsaid control electrode in degenerative sense thereby to provide at saidoutput circuit a substantially uniform decrease of said-componentsamplitudes with respect to the carrier frequency and means for adjustingthe resistance of said resistive element to such a large value that saiduniform degeneration of said components is greatly increased therebygreatly to increase the relative exaggeration of said carrier amplitudewhile maintaining said components at a substantially uniform decreasedamplitude.

2. In the intermediate frequency amplifier of a superheterodynereceiver, a tube having an input circuit and an output circuit eachtuned to the operating intermediate frequency value, means forcompensating for the effect of selective fading wherein the carrierfades relative to 7 its modulation side band frequency components,

said compensating means comprising a pieaoelectric crystal, tuned tosaid intermediate frequency, connected in the tube space current pathbetween the tube cathode and a point of invariable potential, a parallelresonant circuit, substantially free of resistance and tuned to saidintermediate frequency. in shunt with said crystal, voltages of solelysaid modulation side band components and of substantially uniformamplitude being developed across said cystai and resonant circuit, saidinput circuit-being connected between the tube control grid and saidpoint whereby said voltages are degeneratively applied to the controlgrid, a separate resistor, in shunt with each of said crystal andresonant circuit, providing the resistance for said compensation means,and means for adjusting the resistive magnitude of said resistor toincrease the resistance of said compensation means to an extent suchthatsaid degenerative voltages are increased sufficiently to provide suchrelative exaggeration of the carrier frequency as to produce saidcompensation.

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